[css-conditional] Fix missing semicolons at end of IDL blocks.

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=23859

Should make http://www.w3.org/2009/07/webidl-check?doc=http%3A%2F%2Fdev.w3.org%2Fcsswg%2Fcss-conditional%2F&output=html happier.